Breast Milk Is Teeming With Bacteria — That’s Good for the Baby

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A recent series of studies has cast light on a particularly beneficial type of microbe, called bifidobacteria. Groundbreaking work published in 2012 by Omry Koren, who runs the Microbiome Research Lab at Bar-Ilan University in Israel, showed that the microbiome evolves during pregnancy. Over time, it becomes similar to a state that resembles inflammation and metabolic syndrome — a condition associated with heart disease, stroke and type 2 diabetes. In April, Dr. Koren’s team showed that, late in pregnancy, bifidobacteria sense a gestational hormone called progesterone and proliferate in response, as if preparing for the breast-feeding phase.

“We actually think there is a sort of evolutionary mechanism here, which basically causes bifidobacterium to grow faster,” Dr. Koren said. “The fact that bacteria can sense human hormones and react to those hormones, I think that’s pretty fascinating.”

Bifidobacteria consume special sugars found in breast milk, called human milk oligosaccharides, that babies cannot otherwise digest. As a result, these bacteria are abundant in the guts of breast-fed infants but not of formula-fed babies.

Breast milk has long been known to contain human milk oligosaccharides, but the discovery that they’re important for the microbiome to thrive has drawn attention to them. Cows make about 40 different oligosaccharides, and humans make more than 100, although each woman’s breast milk contains a subset of that number.

A woman’s breast milk also contains a unique mélange of hormones, antibodies and bacteria — a brew that presumably evolved to meet the needs of her child. “It’s one of the unique things about human milk that’s really hard to replicate,” Dr. Azad said. “A formula is an attempt at making something that’s one-size-fits-all, essentially.”

Formula manufacturers keep up with the science on breast milk, and they strive to mimic the substance as closely as possible. But in the case of oligosaccharides, they may have jumped the gun: Some companies are adding one of the complex sugars, called 2FL, to their products (and are testing many others). However, about 20 percent of women don’t produce 2FL in their breast milk. If one of these women gives her baby a formula that contains 2FL, the baby receives a sugar that it would not have received naturally, Dr. Azad said: “And is that a good thing? Is that a bad thing? We don’t really know yet.”

Adding just one sugar also may not provide much benefit. Research suggests that only about 10 of the sugars seem to be particularly important for a baby’s health — but only when consumed in combination.

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